1. Field of the Invention
The present invention relates generally to a method of measuring the thickness of a thin plate-like workpiece such as a semiconductor wafer when a grinding tool is pressed against and grinds the surface of the workpiece that is held onto a chuck table for rotation. In particular, the invention relates to a contact type thickness-measuring method in which the thickness of a workpiece is measured by bringing the probes of measurement gauges into contact with two points, the surface of the workpiece and the surface of a chuck table.
2. Description of the Related Art
Some thin plate-like semiconductor substrates which become the materials of semiconductors or electronic components are made of, for example, a single-crystal material such as silicone, or of a compound containing a plurality of elements. Such substrates which were obtained by slicing an ingot of raw material are subjected to chamfering, lapping, etching and other processes. Then, one side or both sides of the substrate are ground to a target thickness. A general method of grinding a substrate involves pressing a grinding wheel against the substrate held on a rotating chuck table while measuring the thickness of the substrate. There is known substrate thickness measurement by use of a two-point contact type thickness measurement gauge which brings the probes of the thickness measurement gauges into contact with respective surfaces of the substrate and the chuck table and determines a difference between both the measurement values as a thickness (see Japanese Patent Laid-Open No. 2000-6018).
When a substrate is ground, a process called “self-grind” is performed in which the surface of a chuck table on which the substrate is placed is evenly ground by grinding machine itself. If the “self-grind” is performed, grinding marks are formed like a radial stripe pattern on the surface of the chuck table. The grinding marks have an asperity of as minute as about 1 to about 2 μm. In order to grind a substrate the probe is brought into contact with the surface of the chuck table and the chuck table is rotated as described above. At this time, if the rotation speed is relatively fast, a phenomenon occurs in which the tip of the probe coming into contact with the chuck table is worn away by the grinding marks.
If the substrate is ground and measured in thickness with the chuck table side probe worn away, then its thickness thus measured is greater than the actual one. If control is exercised such that such an error is compensated for and grinding is performed to provide a target thickness, the substrate is excessively ground as a result to provide a thickness smaller than the target thickness. In particular, the probe on the chuck table side has a high degree of wear since it is located on the outer circumferential portion of the chuck table and the outer circumferential portion is faster in circumferential velocity than the inner circumferential portion. Increased rotation speed makes such tendency noticeable.